1. Field of the Invention
This invention relates to a light alloy piston for internal combustion engines, particularly for diesel engines, comprising an insert which consists of a body of fibers and/or whiskers and is embedded in the cast material adjacent to each piston pin boss.
2. Description of the Prior Art
The increase of the ignition pressures in modern internal combustion engines requires the cast light alloy pistons, usually made from conventional aluminum-silicon alloys, to be designed for a higher strength particularly adjacent to the surfaces in contact with the piston pins. Even when the parts which serve to mount the piston pin in the bosses defining the piston pin bores are designed to withstand high loads, e.g., in that the connecting rod has a width of 35%, the piston pin has a diameter of 42% and the piston pin has a length of up to 82% of the piston diameter and the bearing surface determined by the diameter and the contact length amounts to about 22% of the surface area of the piston head, a maximum combustion pressure from 150 to 170 bars will cause a pressure from 680 to 770 bars to be applied in the bosses defining the piston pin bores. Said values are in excess of the limit of about 650 bars for which a cast light alloy piston can reliably be designed with conventional bosses defining the piston pin bores if an adequate expandible length in excess of 20% of the piston diameter is to be ensured.
In an attempt to retain the design of the inexpensive light alloy piston also for higher loads, it has already been proposed to give that part of the piston pin bore which is nearer to the connecting rod a conically flaring shape (German Patent Publication No. 21 52 462) or to taper the bores adjacent to the inner edge of the piston pin bosses so as to obtain a more uniform distribution of the considerable pressure applied (Zeitschrift 42, No. 10/1981, pages 409-412, Franckhsche Verlagshandlung, Stuttgart). But the change of the stress distribution adjacent to the interface between the piston pin and the piston produces undesired results regarding the stresses arising in other portions of the light alloy piston. For instance, in light alloy pistons formed in the piston head with a combustion chamber recess a stress increase in the pressure-backpressure direction and in approximately the same degree in the direction of the piston pin has been observed in those regions of the rim of the combustion chamber recess which are subjected to the highest gas forces.
In order to permit higher combustion pressures to be taken up without a need for a change of material, it has been proposed to provide a partial fibrous reinforcement preferably at the piston head, in the ring zone, in the bosses and in the skirt (company publication: 75 Jahre Kolbenschmidt, page 47, Kolbenschmidt AG, Neckarsulm, 1985). Whereas fiber reinforced light alloy pistons have better properties from the aspect of mechanical technology than light alloy pistons made from the conventional aluminum alloys, their manufacture is much more expensive.